The sensitivity of a metal detector is dependent not only on the size of the aperture and the type of product you’re running, but also on the types and sizes of metals. For simplicity, all metals can be categorized into three types:
When it comes to types and sizes of metals, test pieces utilize spheres because they are the same shape from any angle. A true contaminant is rarely spherical and will produce a signal dependent on its orientation as it is conveyed through the aperture. The most dramatic example is shown by wire contaminants.
With wire, the signal produced will vary greatly based on the type of metal and its orientation as it passes through the metal detector (see image below). In a worst-case scenario, a wire may produce a signal no bigger than a sphere the same size as the diameter of the wire.
A – Easiest position, biggest signal.
B, C – Hardest Position, smallest signal.
B, C – Easiest position, biggest signal.
A – Hardest position, smallest signal. Flow
In a sense, metal detectors are surface area detectors. They are calculating the types and sizes of metals. This has a considerable impact on determining the size of the metal you’re working to detect and set as your standard. For instance, you might think a 1.0mm Ferrous sphere is half the size of a 2.0mm Ferrous sphere. But a 1.0mm sphere has a surface area of 3.14mm2 while the surface area of the 2.0mm sphere is 12.57mm2. So the 2.0mm sphere will be (in a head to head comparison, not taking into account any product effect or other variables) 4 times easier to detect.
This is another reason that test standards are typically larger for harder metals to detect – like the Stainless Steel. More surface area means it’s easier to detect. More magnetic means it’s easier to detect. HACCP plans and standards are designed and evaluated with these difficulties in mind. An auditor should know that if a company is using the same size for all 3 metals, it’s probable they can be detecting smaller sizes of the ferrous and non-ferrous metals and may want to inquire why the smaller sizes are not being used.
A good Quality Control department will know about these difficulties in regard to types and sizes of metals as they evaluate, test and determine the HACCP plan for each product produced and then purchase the appropriate test pieces for ongoing testing.